1. Field of the Invention
The present invention relates to a semiconductor integrated device and a method for manufacturing the same. More particularly, it relates to a driving device having a high tolerance voltage and being integrated in a semiconductor integrated device for directly driving, for example, a phosphor-fluorescent tube, and a method for easily manufacturing the driving device.
2. Description of the Prior Art
In general, a semiconductor integrated circuit device (hereinafter referred to as an IC device) comprises metal-oxide semiconductor field effect transistors (MOSFETs) driven at a low voltage of, for example, 5 V. Because the operating speed of an N-channel MOSFET (hereinafter referred to as an N-MOS transistor) is higher than the operating speed of a P-channel MOSFET (hereinafter referred to as a P-MOS transistor), N-MOS transistors are usually employed in an IC device.
When a device, such as a phosphor-fluorescent tube used in an electronic calculator, a digital watch, etc., is driven at a high voltage of, for example, 40 volts by an IC device, the IC device should have an output circuit having a high tolerance voltage. Conventionally, an N-MOS output transistor having a high tolerance voltage, is used in the output circuit. The drain of the N-MOS output transistor is directly connected to an output terminal in an open-drain fashion. In the conventional IC device, however, an additional circuit is necessary for driving a device such as a phosphor-fluorescent tube (described in detail later). The additional circuit is provided on the exterior of the IC device. Because of the necessary additional circuit, the number of parts for driving the device is increased and thereby the cost of the system for driving the device is increased. Further, because the drain of the N-MOS output transistor is directly connected to the output terminal in an open-drain fashion, the N-MOS output transistor is off while the device is being driven and is on while the device is not being driven. Therefore, electric power is consumed in the N-MOS output transistor even while the device is not being driven. Usually, the time period during which the device is not driven is longer than the time period during which the device is driven. Thus, in the conventional IC device, there is a problem of large power consumption.